1. Field of the Invention
The present invention relates to an engine exhaust gas purification device.
2. Description of the Related Art
With respect to an engine in which a lean air-fuel mixture is burned, the same applicant has proposed a new type of engine in which an NO.sub.x absorbent is arranged in the exhaust passage of the engine. This NO.sub.x absorbent absorbs the NO.sub.x when the air-fuel ratio of the exhaust gas flowing through the NO.sub.x absorbent is lean, and this NO.sub.x absorbent releases the absorbed NO.sub.x when the air-fuel ratio of the exhaust gas flowing through the NO.sub.x absorbent becomes rich. In this engine, the NO.sub.x produced when the lean air-fuel mixture is burned is absorbed by the NO.sub.x absorbent. The air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent is temporarily made rich before the absorbing ability of the NO.sub.x absorbent is saturated, and at this time, the NO.sub.x is released from the NO.sub.x absorbent. In addition, at this time, the NO.sub.x thus released is reduced (see copending U.S. patent application Ser. No. 66,100 derived from PCT application JP92/01279).
However, this NO.sub.x absorbent has an NO.sub.x absorbing ability which depends on a temperature, and the NO.sub.x absorbing ability of the NO.sub.x absorbent becomes high when the temperature of the NO.sub.x absorbent is within a predetermined range. Nevertheless, in the above-mentioned engine, since the temperature of the NO.sub.x absorbent is not controlled so that it is maintained within the above-mentioned range, a problem arises in that the NO.sub.x absorbing ability of the NO.sub.x absorbent is reduced when the temperature of the NO.sub.x absorbent is above or below the above-mentioned range.